Efforts to render portable apparatuses and electronic devices, already sufficiently scaled down through current fabrication techniques, encounter difficulties in finding portable sources of electrical energy capable of ensuring a prolonged autonomy. The portable sources are batteries with a high energy/volume or weight ratio.
In fact, not withstanding miniaturization and constant reduction of power absorption by current integrated electronic systems, the constant increase of offered functionalities of single portable instruments implies heavy burdens on rechargeable batteries commonly employed to power these portable instruments.
In terms of the limits of the energy/volume and/or weight ratio, even for the most advanced rechargeable batteries being used, attention has shifted onto primary energy converters. In particular, attention is shifted onto the fuel cells capable of converting the chemical energy of an oxidable fuel (either gaseous as hydrogen or liquid as a methanol and other oxidable fuel solutions) into electrical energy in an electrochemical cell.
These cells, commonly referred to as fuel cells, are constituted by catalytic electrodes permeable to the fluid reagent. They are separated by a solid electrolyte, commonly of a proton (H+) exchanging resin which, besides representing the medium for the passage of an ionic current (migration of ions), separates the fuel fed to the negative micro porous catalytic electrode (anode) from the oxygen (air or other oxygen containing mixtures) fed to the positive micro porous catalytic counterelectrode (cathode) of the cell.
Fuel cells could offer an energy/weight ratio greater than that of even the most advanced rechargeable batteries. Their operability as electrical power sources may be considered unlimited given an unlimited availability of fuel that may be stored in relatively large quantities in a small container (even pressurized in case of a gaseous fuel).
Efforts are being made to develop effective micro cell architectures that can be realized on monocrystalline silicon, through modern MEMS techniques of micromachining silicon including the ability of forming cavities by electrochemical preferential erosion of heavily doped domains of the crystal. This is followed by oxidation of the porous silicon residue, and optionally by a chemical leach of the oxidized residue. In general, silicon micromachining techniques have been developed for fabricating transducers, actuators, sensors and other passive components directly on the same chip onto which is integrated the circuit or electronic subsystem employing the transducers, actuator, sensor or the peculiar passive structure formed by micromachining a portion of the monolithic crystalline silicon chip. These techniques are exploited for forming micro machined silicon parts of micro fuel cells.
The following documents: U.S. Pat. Nos. 6,541,149; 6,811,916; 6,558,770; 6,641,948; 5,316,869; 6,627,342; 6,740,444; 6,506,513; 6,589,682; 6,610,433, and published U.S. Patent Applications No. 2003/0022052A1; 2003/0096146A1; 2002/0020053A1; 2003/0134172A1; 2002/0041991A1; 2003/0003347A1; 2004/0058153A1; and the foreign documents WO0069007; DE19914681A; WO 0045457; DE19757320A; JP07201348 and EP-A-1258937 provide a review of disclosed architectures of silicon micro fuel cells.
All these prior art architectures imply the use of distinct dices of micro machined monocrystalline silicon to be eventually joined by wafer bonding techniques to sandwich a film of ion exchanging resin constituting the solid electrolyte of the cell or cells therebetween.
These known architectures are relatively delicate to assemble while providing for a leak proof sealing of the distinct microfluid circuits for the circulation of the fuel and of the oxidant in the respective anodic and cathodic compartments of the cell or cells composing the battery. These difficulties and criticalities increase costs.